Retractable Locking System Driven by Shape Memory Alloy Actuator for Lightweight Soft Robotic Application

Shape memory alloy actuators (SMA) are widely used in robots owing to their flexibility and light weight. However, there is a drawback in that SMAs need to be used as a bundle owing to their lower force density than the motor. In addition, SMAs require continuous current to maintain a contracted state, which results in the overheating of the actuator, causing breakdown and excessive energy consumption. This study presents a new locking system that can mechanically hold the contracted state of an actuator without consuming energy to overcome the disadvantages of a spring-type SMA. The locking mechanism was designed with a ratchet structure and pawl to prevent the actuator from stretching in the opposite direction when locked. In addition, to switch the locking and unlocking states with a single one-way actuation, a bistable retractable mechanism was applied to the system. Repeatability experiments and an analysis of the locking system resolution were conducted to validate the performance of the proposed mechanism. Furthermore, an underactuated finger applied to the proposed locking system was fabricated, and its feasibility is experimentally presented.